In the machining of parts to be used in manufacture, the critical tolerance of machined surfaces must be consistently maintained to produce products which are high in quality and reliability. To machine parts with the necessary precision, a part must be accurately located with respect to a machine tool performing the machining operation. Where high volume production of parts is required, an automated machining center, such as a multiple station boring machine, is frequently used to quickly and efficiently perform several machining operations on a part.
In the past, machines having at least two machine tool stations and a workpiece indexing table and having a single, common base have been used to perform several machining operations on a part or workpiece carried by a fixture. To provide consistent machining accuracy, each component of the machine must accurately radially, axially and circumferentially located with respect to every other component For example, each station attached to the base of the machine must be accurately radially, axially and circumferentially located with respect to each other and the transfer table. Additionally, each workpiece holding fixture must be accurately radially, axially and circumferentially located in its associated station.
In the typical operation of a machine of this kind, a fixture, already accurately located with respect to the indexing table carrying it, first receives a workpiece which is accurately located in the fixture. After clamping the workpiece in the fixture, the table is accurately indexed to deliver the workpiece to a first machine tool station. After all machining operations are performed at that station, the table accurately indexes the workpiece seriatim to each succeeding station in which machining operations are performed. Once all machining operations are completed on the workpiece, the table is indexed back to a station where the finished workpiece is removed from the fixture and another workpiece is transferred into the fixture to be machined.
A drawback of this prior art machine is that it is sensitive to any mislocation of any component involved in the machining process. For example, if the workpiece, fixture, indexing table or any machine tool is even slightly out of location, the workpiece may be machined out of acceptable tolerances resulting in the costly scrapping of the piece. This design is also subject to cumulative error because it requires that the workpiece be accurately located in the fixture, the fixture be accurately located on the table, and the table be accurately located and indexed with respect to each machine tool station, etc. Any single error in locating any component with respect to any other component may result in a potentially defect-producing machining error. More over, if two or more errors in location exist, the errors may aggregate to increase the amount that the workpiece may be out of tolerance raising the probability that the part will have to be scrapped.
The cumulative error problems commonly associated with this design also limit the machining accuracy, quality and reliability of the machine to produce a properly machined workpiece. This design also lacks flexibility and may be of limited cost-effectiveness by sharing a common base because index table or machine tool station obsolescence or failure may require the costly replacement of the entire machine.
The problems commonly encountered with this design require that each component of the machine be extremely accurately made and located as any location error will result in the costly scrapping of a defectively machined workpiece. Even worse, the incorporation of out-of-tolerance machined parts into a finished product will result in a product of inferior quality and reliability requiring warranty work to be performed, increasing customer returns and lessening profits Such an inflexible design also increases operation and replacement costs when any machine station or index table is rendered obsolete or unusable necessitating replacement of the entire machine.